Capacitors do have a shelf life and AFAIK tend to leak after 10 to 15 years of use. You may want to source a new one on Ebay to have as a spare, as the one given to you may not have a lot of life left. My .02
You have to allow that eBay of any effective sort is not an option for Michael (
@Suzuki4evr ), he being in a country where eBay just isn't.
That said, AC motor starting capacitors are not expensive, and will certainly be available from the right store. It's OK to use used ones, These things can be very long lived. They do die from very old age if they leak, or the electrolyte dries up, or the oxide deteriorates.
How they work
Now to clear up some misconceptions about AC electrolytic capacitors. It is
VERY important to only use capacitors specifically marked as AC types. This is because, values like 40uF (40 micro-Farads) can only be built in a thing that small by making the insulating gap between the capacitor plates incredibly thin. It is is, in fact, a layer of aluminium oxide that is built up by electrolysis, with a wet electrolyte.
These types of electrolytic capacitor are polarized. The DC sort can only be used connected the correct way around. You see them in any electronic circuit.
So what makes a "AC" motor capacitor?
Hidden inside the can is a construction that is actually
two capacitors connected in series, with one reversed, so to have polarity connected "back-to back". In normal use, a DC electrolytic capacitor always has the voltage replenishing and preserving the oxide layer. If connected the wrong way, the leakage current is dissolving the oxide, and eventually it breaks down, and the capacitor blows up. I have had this happen on a tiny 100uF capacitor. It was loud, and it made a huge mess!
With AC connected capacitors, in series, the actual capacitors in there are double the value of the final series combination.
2 of 80uF in series becomes only half that, i.e. 40uF total, but the benefit is the voltage breakdown rating is doubled.
With AC 50Hz or 60Hz current, at any time in the cycle, one of the series pair is the "correct" way round, there is not time for the other to have it's oxide break down. For 50Hz, only 20mS cycle time, so only 10mS being "reversed".
When there are two capacitors in the can.
It's actually four, but connected internally in series, so you only see two.
What if there are three connections?
Some capacitor connections to motors use the outer of the can, insulated, to be the common connection used by two capacitors, such as might be for "starting" and "running" capacitors. More likely, if not insulated, the outer (bare) can is to a mounting bolt. This would still be to ungrounded parts of the motor wiring. Otherwise, its to a frame safety ground. This is stuff one needs to take care over.
"Micro" Farads?
Yes - a millionth of a Farad. (named after Michael Faraday)
A whole Farad is a capacitor that would get up to only 1 Volt, even from running a current of a 1 whole Ampere into it for 1 whole second.
It is, in fact, a large amount of energy. That is what you call a "supercapacitor", as used in Formula 1 cars.
40uF at 450V is a decent size, small enough physically. It gets up to hundreds of volts in a quarter of a cycle, so 5 milliseconds in RSA, or 4.16mS if in the USA, where mains is 60Hz
Why with a motor?
This is to be able to drive single phase motors. The coils in the motor can make a magnet field between them, but they need another set of coils made rotated 90 degrees, to continue to pull on the rotor when it has turned some. You can connect the mains live power to one coil set, but you need another mains power running out of phase by 90 degrees relative to the first, to connect to the second set of coils.
The way the "second" power is derived is to put it through a AC capacitor. It initially charges up, and then, when in series with the motor coil, makes the "other" AC power that is phase shifted as needed. The motor will turn, and in the correct direction.
I know all this is a bit of a rough, qualitative explanation, but I hope it helps. Know that 5% is quite accurate. The motor will likely still work OK if the capacitor value is even 20% wrong, but it helps if the value is "higher" rather than lower. Also, most motors do not have a special separate starting capacitor. One often sees capacitors in motors with one connection not used. Note also the high voltage rating. Do not use capacitors in mains motor circuits with ratings less than the 450V norm.
